Only 50% of surgical patients with acquired esotropia achieve good eye alignment with one surgery. Evidence indicates that with preoperative use of prisms, good eye alignment can be achieved with one operation in approximately 85% of surgical patients. A multicenter, randomized, prospective, clinical trial with careful attention to standardization and unbiased outcome evaluation is proposed to determine if: 1. The surgical result in patients with acquired esotropia is better if they have preoperative prism adaptation. 2. If patients who respond to be prism adaptation test are more accurately corrected by operating for the prism adapted angle or the original angle of deviation. 3. If there are clusters of input variables, (e.g. age, size of deviation, sensory status, refractive error, or AC/A ratio) that help pradict which patients are more likely to respond to the prism adaptation test. Two-thirds of the patients will be randomly selected to go through prism adaptation prior to surgery. One-third of the patients will have surgery based on the amount of crossing measured using routine examination techniques. Of patients who respond to the prisms, one-half will have surgery based on the amount of prism required to stabilize the deviation; the rest will have surgery based on the amount of crossing originally measured. If the prism adaptation test leads to more successful surgery, its adoption can have a significant effect in decreasing the cost and morbidity for the treatment of acquired esotropia. Approximately 2,500 reoperations for acquired esotropia are performed each year in the U.S. If these are reduced by two-thirds by using the prism adaptation test, with each operation costing between $2,000 and $3,000, the dollar saving would be between $3.2 million and 4.8 million per year. Morbidity would be reduced with fewer operative complications occurring, less time lost from school and work, and less psychological trauma on the part of the patient, family, and physician.